This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Rhesus macaques were immunized with three different strains of single-cycle SIV that express envelope glycoproteins modified to lack structural features that interfere with the development of neutralizing antibodies to determine if enhancing envelope-specific antibody responses might improve the extent of protection afforded by this vaccine approach.